Coffee is not only a hot morning beverage, but in Castle, it serves as a way for Richard Castle and Kate Beckett to show that they care for each other. Early in the series, after Castle comments on how badly the coffee made at the precinct tastes (in Hell Hath No Fury he says "it tastes like a monkey peed in battery acid"), he buys the Homicide Division a high-end cappuccino/coffee maker. He and Beckett spend a fair amount of time talking around the coffee machine about their latest case. It becomes a time when they get to be by themselves in the division break room away from the business going on at their colleagues' desks.
In most episodes, beginning with Little Girl Lost (episode 1.9) when Castle surprises Beckett with her favorite cup of coffee and a bear claw pastry, Castle comes into work at the precinct in the morning with a cup of coffee for himself and Beckett, just as a little sign that he cares about her. This gesture takes on more significance when Beckett meets and begins dating Detective Tom Demming. In Den of Thieves (episode 2.21), Demming beats Castle to bringing Beckett coffee and at the end of the episode, Castle watches as Demming has seemingly replaced him as the one Beckett shares coffee with around the coffee machine he bought. In A Deadly Game (episode 2.24), when Beckett makes it clear to Castle that she and Demming are a couple, Beckett is surprised and visibly disappointed when Castle arrives one morning with only a cup of coffee for himself.
In A Deadly Affair (episode 3.1), after spending the summer apart from each other, Beckett grins widely when Castle arrives for work and places a cup of coffee in front of her on her desk as if nothing had happened.
In 3XK (episode 3.6) after Beckett saved Castle and Detective Ryan from the Triple Killer, Beckett brings Castle a cup of coffee as he sits by himself trying to get over the frustration and guilt of knowing that the Triple Killer got away.
When actress Natalie Rhodes shadows Beckett as research into the role of Nikki Heat for the movie version of Heat Wave in the episode Nikki Heat, Beckett is very put off when Natalie steals the cup of coffee Castle brings in for Beckett.
In the season 4 finale, Always, Castle makes a plea for Beckett's heart by saying "Every morning I bring you a cup of coffee just so I can see a smile on your face because I think you are the most remarkable, maddening, challenging, frustrating person I’ve ever met." Once Castle and Beckett are in a relationship, on her first day back at the precinct after serving a suspension, Castle does not bring Beckett her usual cup of coffee in an effort to keep things casual between them. Beckett disagrees, thinking that it will be all the more noticeable, a fact that Detective Esposito notices right away and for which he chastises Castle.
In one of Castle's Nikki Heat books, Frozen Heat, he even references Beckett's favorite type of coffee (grande skim latte, two pumps sugar-free vanilla) as being Nikki Heat's favorite as well. At one point, a colleague of Heat's tries to get her coffee and fails miserably at remembering her favorite kind.